In recent years, apparatuses that digitally deal with image information in conformity to a system such as MPEG (Moving Picture Experts Group) in which compression is carried out by using redundancy unique to the image information by an orthogonal transform such as a discrete cosine transform and a motion compensation for purposes of highly efficient information transmission and accumulation at that time have been spread in both information distribution by broadcasting stations or the like and information reception in general households.
In particular, MPEG2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) is defined as a general-purpose image coding system, is a standard covering both an interlace scanning image and a non-interlace scanning image as well as a standard resolution image and a high-resolution image, and is currently widely used for broad applications for professional use and consumer use. Since the MPEG2 compression system is used, for example, it is possible to realize a high compression rate and a satisfactory image quality by allocating a code amount (bit rate) of 4 to 8 Mbps to the interlace scanning image at the standard resolution having 720×480 pixels and 18 to 22 Mbps to the interlace scanning image at the high-resolution having 1920×1088 pixels.
MPEG2 has mainly been intended for high image quality coding adapted to broadcasting purposes but does not correspond to a coding amount (bit rate) lower than MPEG1, to elaborate, a coding system at a higher compression rate. With the spread of portable terminals, it is conceivable that needs for such coding system will be increased from this time, and standardization of MPEG4 coding system has been established to correspond to this. With regard to the image coding system, the specification is approved as ISO/IEC 14496-2 for an international standard in December, 1998.
Furthermore, in recent years, for purposes of the image coding for television meetings at first, setting of a standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) has been progressed. H.26L demands a higher computation amount for its coding and decoding as compared with the conventional coding system such as MPEG2 or MPEG4, but it is known that a higher coding efficiency is realized. Also, currently, as one of the activities for MPEG4, standardization based on this H.26L for realizing a higher coding efficiency is conducted by also taking in functions that are not supported by H.26L as Joint Model of Enhanced-Compression Video Coding.
As a schedule for the standardization, international standards in the names of H.264 and MPEG-4 Part10 (Advanced Video Coding, which will hereinafter be denoted as AVC) are established in March, 2003.
Incidentally, in the AVC, a hierarchical structure based on a macro block and a sub macro block is stipulated as a processing unit for the coding (coding unit). However, a setting of this macro block size as 16 pixels×16 pixels is not optimal to a large image frame such as UHD (Ultra High Definition; 4000 pixels×2000 pixels) which may be a target of a next generating coding system.
In view of the above, in HEVC (High Efficiency Video Coding) corresponding to a PostAVC coding system, a coding unit (CU (Coding Unit)) is stipulated as the coding unit instead of the macro block (for example, see NPL 1).
In the case of the HEVC, in a sequence parameter set SPS (Sequence Parameter Set), a largest size of the CU (LCU (Largest Coding Unit)) and a smallest size (SCU (Smallest Coding Unit)) are stipulated by two syntax elements called log 2_min_coding_block_size_minus3 and log 2_diff_max_min_coding_block_size.
Also, a prediction unit (PU (Prediction Unit)) is stipulated in this SPS. This PU indicates a size (processing unit) of an intra prediction block in an intra prediction and indicates a size (processing unit) of a motion compensation block in an inter prediction. Furthermore, a transform unit (TU (Transform Unit) is also stipulated in the SPS. This TU indicates a size (processing unit) of an orthogonal transform.
In the case of the inter prediction PU (Inter Prediction Unit), four types of sizes including 2N×2N, 2N×N, N×2N, and N×N can be set with respect to the CU having a size of 2N×2N.
Incidentally, in the SPS, inter_4×4_enable_flag corresponding to flag information related to this inter prediction unit size is defined. In a case where a value of this flag is “0”, use of a 4×4 block size is prohibited in the inter prediction unit in the image compression information.